The present invention relates to a gas sensor, in which a gas sensitive film for detecting a gas is located on a membrane supported by a semiconductor substrate, and to a method for manufacturing the gas sensor.
JP-A-58-102144 and JP-B2-2582343, the corresponding US patent of which is U.S. Pat. No. 5,545,300, disclose gas sensors of this kind. The sensors disclosed in the publications include a membrane, which is located at the bottom of a recess that is formed in a semiconductor substrate by etching the semiconductor substrate. The membrane includes a gas sensitive film made of a material such as tin oxide (SnO2) and indium oxide (In2O3) and a heater that generates heat when being electrified. The sensors also include a pair of gas-sensitive-film extension electrodes, which are respectively connected to the gas sensitive film, and a pair of heater extension electrodes, which are respectively connected to the heater.
In the sensors disclosed in the publications, the heater and the extension electrodes are made of a metal having a high melting point such as platinum because the temperatures of the heater and the extension electrodes becomes extremely high on the membrane in operation. However, metals having a high melting point such as platinum may become a pollutant in the fabrication processes of ordinary semiconductor microchips such as a C-MOSFET, so it is preferred to avoid using the metals to make the manufacturing process of a gas sensor compatible with that of ordinary semiconductor microchips.